Method for controlling back pressure in electrically-operated injection apparatus

Abstract

A method for controlling back pressure in an electrically-operated injection apparatus wherein rotation and forward movement of an injection plunger within an injection heating tube are carried out by a first electric motor which provides a driving force, wherein backward motion of the injection plunger during the charging of material presses against a screw threaded rotatable part and is thereby converted into rotational motion of the threaded rotatable part, and wherein the rotational motion is controlled to control back pressure of the injection plunger. A second electric motor is used for controlling the back pressure on the injection plunger by controlling rotation of the threaded rotatable part. The backward force exerted on the injection plunger or the direct pressure of the molten material within the injection heating tube are electrically measured. By driving and controlling the second motor through feedback control, the back pressure is made to coincide with a set, preselected back pressure value.

What is claimed is: 
     
       1. A method for controlling back pressure in an electrically-operated injection apparatus, the method comprising the steps of: introducing a material to be molded into a tube having an injection plunger therein and rotating the injection plunger to move the material forward in the tube, the injection plunger moving in a backward direction with respect to the tube in response to a back pressure developing from the material being moved forward in the tube;   coupling the injection plunger to a rotatable part in a manner which is effective to rotate the rotatable part in response to the backward movement of the injection plunger; and   sensing the instantaneous value of the back pressure and applying to the rotatable part a braking force which is responsive to the instantaneous back pressure and which is effective to regulate the backward movement of the injection plunger for causing the back pressure in the material to coincide with a preselected back pressure value.   
     
     
       2. The method of claim 1, wherein the step of sensing the instantaneous value of back pressure includes the step of sensing the strain imparted to a portion of the apparatus which receives thrust produced in the rotatable part in reaction to the back pressure exerted on the injection plunger and producing a strain signal representative of the strain. 
     
     
       3. The method of claim 2, including the step of receiving the strain signal and a back pressure set signal which represents the preselected back pressure value and combining the signals to produce a feedback output which is operative for controlling the braking force. 
     
     
       4. The method of claim 3, in which the feedback output is applied to a back pressure control motor which is coupled to the rotatable part and which is operable by the feedback output to apply the braking force to the rotatable part. 
     
     
       5. The method of claim 3, comprising the step of adding the strain signal and the back pressure set signal to produce the feedback output. 
     
     
       6. The method of claim 1, further comprising coupling the injection plunger to a gear which is rotatable independently of the rotatable part and rotating the gear via a transmission which is driven by a main driving electrical motor. 
     
     
       7. The method of claim 1, wherein the rotatable part comprises a screw shaft and wherein the screw shaft and the injection plunger are generally disposed in line with one another. 
     
     
       8. The method of claim 7, further comprising the step of driving the screw shaft, during a molding phase of the apparatus, in a direction for causing forward movement of the injection plunger in order to drive the material from the tube into a mold. 
     
     
       9. The method of claim 4, wherein the back pressure controller motor is exclusively operated for the task of braking the rotation of the rotatable part. 
     
     
       10. The method of claim 1, comprising sensing the instantaneous back pressure directly at the material in the tube.